When designing transducers such as pressure sensors, acceleration sensors, microphones, or loudspeakers, it may be typically desirable to achieve a high signal-to-noise ratio (SNR). The continuous miniaturization of transducers may pose new challenges with respect to the desired high signal-to-noise ratio. Microphones and to some extent also loudspeakers that may be used in, for example, mobile phones and similar devices may nowadays be implemented as silicon microphones or microelectromechanical systems. In order to be competitive and provide the expected performance, silicon microphones may need high SNR. However, taking the condenser microphone as an example, the SNR may be typically limited by condenser microphone construction.
The issue of the limited SNR that can be achieved with current designs of condenser microphones, especially when implemented as a MEMS, can be explained as follows. A condenser microphone may typically comprise a diaphragm and a backplate that may serve as a counter electrode. The sound may need to pass through the backplate and as a consequence, the backplate may be typically perforated. Note that the backplate may need to be perforated even in those designs in which the backplate may be arranged behind the diaphragm (i.e., at the side of the diaphragm facing away from the direction of arrival of the sound), because during operation the diaphragm may push some of the air in the volume between the diaphragm and the backplate through the perforated backplate to a backside cavity. Without the backside cavity and the perforation in the backplate, the volume between the diaphragm and the backplate might act like a very stiff spring and hence might prevent the diaphragm from significantly vibrating in response to the arriving sound.
A different design of capacitive microphones may use a so-called comb drive where the diaphragm and the counter electrode have a plurality of interdigitated comb fingers at a lateral circumference of the diaphragm. These comb sensor microphones may have reduced noise due to the missing backplate. Still there may be a fluidic element of noise in between the interdigitated comb fingers.